1. Field of the Invention
This invention relates to an armature for conducting very large currents between two parallel rails as it slides along the rails and more particularly it has application to such an armature for apparatus used in the electromagnetic propulsion of projectiles.
2. Description of the Prior Art
In the electromagnetic propulsion of projectiles, a very large dc current, on the order of several hundred thousand amperes, is injected into the breech end of a pair of parallel rails. A projectile which is in sliding contact with the rails is driven toward the muzzle end of the rails where it is ejected at a very high velocity, on the order of several kilometers per second, by the electromagnetic forces generated by the very large current. In many of these projectile launching assemblies, the projectile is provided with an armature which conducts the current between the rails. Originally, the projectile itself was a solid conducting body which served as the aramature. Subsequent projectiles included a separate armature, made up of "leaves" of conductive material affixed to the rear of the projectile. The leaves were stacked in the direction of movement of the projectile with each leaf bridging the gap between the rails. The purpose of the laminated armature was to provide better sliding electrical contact between the armature and the rails. Electrical contact was enhanced by making the laminations of resilient, conductive sheets bent about an axis transverse to the direction of armature movement in a chevron configuration so that when the armature was placed between the rails, the ends of each leaf trailed toward the breech end of the launched end and were biased against the adjacent rails. In some configurations the center portion of each lamination was perpendicular to the rails with just the ends trailing rearward.
Experience has shown that with these prior art armatures, the current is concentrated in the corners of the armature adjacent the breech end of the rails. The higher current density at these points causes them to become hot spots. While the multiple leaf configuration reduces this current concentration somewhat, true uniform current distribution among multiple leaves cannot be achieved (at any armature velocity) no matter how thin the individual leaves are made if they are all identical. In fact, the discovery that practically all of the current was carried by the rearmost leaf, led one researcher to discard all of the chevron shaped leaves except one. This single leaf was then laminated from sheet oriented parallel to the rail faces to provide the multifinger contact between the rails and the armature.
The arrangement of a conductive armature driven down the gap between two parallel conducting rails is also used in some electromagnetic propulsion systems as a firing switch for injecting the very large dc current into the launcher rails. The launcher rails are connected to one rail of the switch on either side of a non-conducting section of the switch rail. Then, as the switch armature, which is driven by the very large dc current down the switch rails, passes the non-conducting section of the one switch rail, the current is commutated into the launcher rails. This switch armature therefore carries the full current applied to the launcher rails and is subjected to the same heating problems associated with current concentration at the rear corners as is the projectile armature. The situation is of even more concern in the case of the switch armature since it is intended to be used over and over again for firing the launcher unlike the projectile armature which ordinarily need survive only one shot.
Rail switches have also been proposed for use as a power switch in an electromagnetic propulsion system in which a kinetic energy storage device, such as a homopolar generator, applies a very large dc current to an inductive energy storage device which is in series with the firing switch. When firing is completed the homopolar generator is removed from the circuit by the power switch and the inductor is crowbarred across the firing switch to dissipate the stored inductive energy. The armature of this rail type switch also carries the full system current and is intended to be used repeatedly so that excessive heating at the breech corners is undesirable.